Echo is the presumed god of the Echoplane . She is mentioned in the reverse lyrics of Prologue III (Echodeath). The lyrics state that "The God, Echo, cried" at 12:12, following what is believed to be the destruction of the Echoplane.
In Greek mythology, Ἠχώ (Echo) was an Oread (a mountain Nymph) who loved her own voice.
There are 3 stories surrounding Echo which all have certain links to love, yet another possibility that the '3 parts to love' could be found in deciphering the stories of Echo. Detailed information on Echo can be found here.
Echo and Hera
In short, Echo and Hera were not in love directly but the fact that Echo decided to distract Hera from Zeus's antics with the other Nymphs by leading her away and flattering her, along with telling her long and entertaining stories. Hera was displeased when she found out that Echo's flattery had all been a deceit to allow Zeus to do as he please. Hera put a curse on Echo so she could only repeat the last words of the people she heard. (This could be something surrounding the first of the three parts to love that are mentioned in the lyrics of Bōsōzoku Symphonic's 4th Movement ).
Echo and Narcissus
There are many different ways the stories of Echo and Narcissus are told but the most notable are listed below.
In this version, it appears that Narcissus has no heart and is unable to fall in love with anybody. Echo falls deeply in love with Narcissus and followed him everywhere. When Narcissus got lost, he called out for help. He then shouted out 'Is anyone here?'. Echo replied 'Here, here, here' (Note the fact that she has repeated the last word of his sentence three times). When Echo revealed herself to Narcissus and told him she loved him by hand movements, Narcissus became angry and told her that he did not love her.
Echo became so upset she prayed to Aphrodite for death, which she was granted but Aphrodite kept her voice which haunts valleys and mountainous areas to this day. Another ending to this version of the story involves Echo spending the rest of her life alone, crying until nothing apart from her bones and voice remain.
Yet another involves Narcissus viewing his own reflection in a still pool of water and being overcome by his own beauty. Whenever he spoke he would hear Echo repeating his words around him. It then ends, either with him starving to death or killing himself with his hunting Knife, both creating a Narcissus flower, either from his withered corpse or from his fresh blood. (The second part to love could revolve around these stories as well).
Echo and Pan
This story involves a Lecherous God named Pan who wanted to be with Echo but as Echo did not like the love of any man, this caused Pan to become angry. In view of this Pan had Echo killed and split into many pieces which were spread about the world but they were collected by Gaia. Another version states that "Echo's voice was not collected, so it remains all over the world and still says the last words that other people have said". (This story may feature another part of love, although this is unclear).